"We just noticed a vast number of dead bodies: old women, old men, babies, infants that were dead or wounded," said Thompson, who was 24 at the time.

Thompson and his crew, 19-year-old gunner Larry Colburn and crew chief Glenn Andreotta, 18, flew closer to the ground and what they saw there confirmed their fears. They watched a fellow American shoot a Vietnamese woman at point-blank range.

Thompson landed his helicopter, determined to stop the killing. Then he and his crew did something that could have cost them their own lives or military careers: They turned their guns on fellow Americans and ordered them to stop shooting.

"This was taking advantage of very helpless, defenseless people, and it's not good," said Thompson. "It's not the American way - wasn't my way, that's for sure."

Once the shooting stopped, Thompson did things his way. He had seen villagers run from American troops and take cover in a bunker, and was determined to save them.

But some soldiers were not through killing. When Thompson told the troops he was going to help those in the bunker, one soldier offered to get them out - with a hand grenade.

"I told them to just hold their troops because I [thought] I could do better than that," said Thompson. "Thank God that everybody played it real cool then and they just held their ground."

The frightened villagers at first were reluctant to come out, but they recognized Thompson as the soldier who had stopped the carnage.

As they emptied the bunker, Thompson realized it had hidden more villagers than he had thought and he began to realize the magnitude of the situation.

He had the wounded airlifted to a hospital, but the damage already had been done. American soldiers had already killed 504 Vietnamese by the time he intervened.

Though Thompson, Colburn and Andreotta could not undo the My Lai Massacre, they likely prevented the killing from spreading to nearby villages.

"My Lai was a very bad situation - one that I hope and pray never happens again, but there's some good that came out of it," Thompson said in a CNN interview. "If we can have any of our younger generation, our college students or whatever, high school, stop and think sometime in their life, 'This ain't right. I'm not gonna do it' and put their foot down and go with what's in their heart, it'll be worth it in the long run."

"It is absolutely the responsibility of every U.S. service member, if they see inhumane treatment being conducted, to intervene to stop it. As an example of how to do it if you don't see it happening but you're told about it is exactly what happened a couple weeks ago. There's a report from an Iraqi to a U.S. commander that there was possibility of inhumane treatment in a particular facility. That U.S. commander got together with his Iraqi counterparts. They went together to the facility, found what they found, reported it to the Iraqi government, and the Iraqi government has taken ownership of that problem and is investigating it. So they did exactly what they should have done."